One of mine does this, and it's grim. He does it mainly at night when he wants biscuits and he gets like real up in your face, pummeling, purring loudly, won't settle, drips on your face, licks a lot too

I read it was in this instance him wanting food and just getting overly excited/happy. I try and make sure he has some biscuits before lights out but still occassionally, dribble dribble